Wave power has a large potential of becoming cost efficient since the energy density in ocean waves is very high (approximately 1000 times higher than in the wind), this allowing small wave power plants in relation to the capacity thereof. Furthermore, wave energy is more predictable than for instance wind power since waves are built by the wind during a long period of time and then continue as swell also after the wind has subsided. This results in slow variations in the average energy content of the waves, which gives system advantages when wave power plants are connected to the general electric power distribution network.
A reason why there are, in spite of this potential, so few competitive solutions today is that wave energy is difficult to master. The ocean is a rough environment with high material stress. In stormy weather the energy levels can be a 100 times higher than normal. The wave motion is oscillating and has never ceasing variations in height, length and time period (velocity) from wave to wave, this giving large variations in the energy absorbed by a wave power plant. For direct driven operation, i.e. when the generator in the wave power plant is driven according to the momentary movement of the wave, this results in a low utilization of the power plant, i.e. the so called capacity factor takes a low value. The power of the generator shifts between zero and a top level twice every wave period. The top level may also change very strongly from wave to wave. The general electric power distribution network requires relatively stable levels, both in delivered power and voltage, this resulting in that the electric control systems for this kind of wave power plants must, after the generation, make the levels of these quantities more even. Also, the uneven levels result in a costly over-dimensioning of the total electrical system of a wave power plant in order to obtain a proper handling of the top power levels.
To make wave power competitive a wave power plant is required that can efficiently absorb the wave energy at the same time as the motive force applied to the generator is levelled or equalized so that a higher capacity factor is obtained. Also, a low system complexity and an efficient use of components are required. Moreover, the structure of the wave power plant must be storm proof and have a long life-time and low operational and maintenance costs that can be achieved by a construction allowing long service intervals and including wearing parts that can be easily accessed.
Wave power technology has been developed for a long period of time but up to now it has not been possible to arrive at a method and a design of a wave power plant, where it has been possible to combine the necessary properties as described above.
A frequent method of capturing the energy of water waves is to use the vertical movement of the water. Installations that use such technology are sometimes called “point absorbers”. One method of using the vertical movements comprises a buoy having a bottom foundation and an anchor wheel. The bottom foundation is firmly positioned on the sea-floor and is connected to the buoy which follows the ocean surface, i.e. the wave movements. When the surface rises and thereby lifts the buoy, a motive force is created which is converted to a rotational movement by a driving bar connected between the foundation and the buoy or by a wire or chain which runs over an anchor wheel journalled for rotating at the buoy or in the foundation and which is at an opposite end connected to the foundation or the buoy, respectively. The motive force increases due to the increased motion speed of the waves when the wave height becomes higher. The rotation direction and speed of an anchor wheel, if such a wheel is used, is directly dependent on the vertical direction and motion speed of the waves. However, this is not optimal for coupling a conventional generator to the anchor wheel to produce electric energy.
In order to make a wave power plant driving a conventional rotating generator efficient, the vertical movements of the waves must be converted into a unidirectional rotational movement, and the rotation speed of an electric generator connected to the transmission must be stabilized. In a device, as described above, using a driving bar, wire or chain, which is secured to the bottom of the sea or in a frame structure and which runs along or over an anchor wheel journalled in a buoy, this problem can be solved in the following way. When the buoy is lifted by a wave, a motive force over the anchor wheel is produced. Thereupon, when the wave falls, an anti-reverse mechanism is disengaged and the anchor wheel is rotated backwards by a counterweight. Then, the motive driving is only active during the rise of the wave and completely ceases when the wave sinks, this not being satisfactory. Attempts have been made to reverse the rotation direction, so that an electric generator driven by the anchor wheel is driven by the counterweight in the same direction also when the wave sinks. It has also been attempted to reverse the rotation direction of the generator. However, changing the rotation direction of a mechanical transmission or of the generator twice in every wave period results in heavy mechanical wear. Even though the rotation direction can be made unidirectional by the transmission, the rotation speed follows the speed of the vertical movement, this causing the power output from the generator to vary according to the speed of the wave movements. This gives to a low capacity factor and high attenuating effects since the mass of the generator all the time must alternately be accelerated and decelerated. In order to make the motive force and rotation speed of a generator more even using a mechanical transmission multiple buoys can cooperate, a phase shift existing between the buoys. However, this only works optimally in the case where the buoys are evenly distributed over a wave period, which very seldom occurs since the length and the speed of the waves always vary. Also, the transmission system becomes more complex and hence hydraulic mechanisms are frequently used in systems of this type. However, hydraulic devices results in complex systems having large transmission losses.
A wave power plant of the type described above is disclosed in the published French patent application 2869368, which comprises a floating platform or buoy. Lines run over pulleys at the buoy, one end of the lines being attached to the bottom and the other end carrying a counterweight. The rotation of the pulleys is transferred to generators. The rotation speed and the power output from the generator vary according to the movements of the waves. A similar wave power plant is disclosed in U.S. Pat. No. 4,242,593, which drives a wheel or pulley in the buoy only when the buoy is rising. A gearbox is provided for gearing up the rotation speed of the wheel or pulley in the buoy to make it suited to be used for driving a generator. In U.S. Pat. No. 5,889,336 and the published Japanese patent application 11-6472 a similar wave power plant is disclosed that includes a chain which is at one end attached to a bottom foundation end and has at its other end a counterweight. The chain passes over a chain pulley in a buoy. The chain pulley is connected to a generator through a directly acting transmission, which is arranged to make the generator always rotate in the same direction. The rotation speed depends on the speed of the vertical movement of the buoy.
A wave power installation of a somewhat different type is disclosed in U.S. Pat. No. 4,241,579. A driveshaft is mounted to be elevated and lowered between the water surface and the bottom. A number of buoys are by lines connected to counterweights and the lines pass around the common driveshaft for driving it only when the respective buoy has an upward movement. In the published British patent application 2062113 a wave power plant is disclosed including a plurality of different drive mechanisms, each one of which comprises a buoy and a counterweight/bottom foundation/additional buoy and which act on a common driveshaft through one-way couplings. In the published French patent application 2339071 a buoy is used, which is connected to one end of a chain and by the chain drives a driveshaft placed above the water surface to rotate. The other end of the chain carries a counterweight, which is also placed above the water surface. The connection to the driveshaft is of a unidirectional type and the driveshaft may be driven by several such buoys through chains.
In the published International patent application WO 2005/054668 a wave power plant including a buoy which is attached to an end of a line is disclosed. The other end of the line is more or less wound around a drum placed on the bottom of the sea. The drum is connected to a return spring and a generator and drives the generator in both the rising and sinking movements of the buoy. In the wave power plant according to the published International patent application WO 03/058054 the buoy acts as an winding drum for a line, the lower end of which is connected to a bottom foundation. A return spring, a gear up mechanism and a generator are arranged inside the buoy. The generator is driven in both the rising and sinking movements of the buoy.